diff --git a/scsh/time1.c b/scsh/time1.c
index bfc0e7c..228f2b3 100644
--- a/scsh/time1.c
+++ b/scsh/time1.c
@@ -174,10 +174,12 @@ scheme_value time2date(int hi_secs, int lo_secs, scheme_value zone,
 #ifdef HAVE_GMTOFF	
 	*tz_secs = d.tm_gmtoff;
 #else
-	{ char **oldenv = environ;			/* Set TZ to UTC     */
+	{ struct tm dcopy = d;
+	  char **oldenv = environ;			/* Set TZ to UTC     */
 	  environ=utc_env;				/* time temporarily. */
 	  tzset(); /* NetBSD, SunOS POSIX-noncompliance requires this. */
-	  *tz_secs = mktime(&d) - t;
+	  dcopy.tm_isdst = 0;
+	  *tz_secs = mktime(&dcopy) - t;	/* mktime() may mung dcopy. */
 	  environ=oldenv;
 	  }
 #endif